A speed-type internal combustion engine, typified by a gas turbine, is a machine in which a working fluid is accelerated by energy generated by combustion, wherein the working fluid flows through a combustor at a high speed. If the temperature and pressure of the working fluid as well as the mixture ratio between the oxidizing agent and fuel are kept constant, flames can be formed, propagated, and maintained in a stable manner.
However, in practice, it is extremely difficult to keep these working fluid conditions constant. Discrepancies arise in the pressure and temperature, and both regions of high fuel concentration and regions of high oxidizing agent concentration arise in the combustor. Such discrepancies change progressively over time.
Therefore, the flame propagation speed fluctuates according to the mixture ration of the fuel and oxidizing agent, and discrepancies arise in the combustion temperature and the ease of ignition. These discrepancies induce combustion oscillation and flame extinguishing.
In view of this, the following measures are commonly used in order to form, propagate, and maintain flames in a speed-type internal combustion engine in a stable manner.
(1) In order to increase the combustion speed, the fuel and oxidizing agent are mixed, the objective being a concentration mixture ratio of about 1.1 times the stoichiometric mixture ratio.
(2) In order to take advantage of the rate of turbulent combustion being higher than the rate of laminar combustion, a turbulent flow or swirl flow is formed in the combustor. For example, in a gas turbine, a recirculation zone where a bulk (branch flow) occurs is created in the combustor by the formation of a swirl flow by the swirler and by the introduction of an oxidizing agent from the side surface of the combustor. A frame holder is also provided in the location of the combustor or afterburner in a jet propulsion engine.
(3) Two-stage combustion is performed by diffusion combustion using a pilot burner.
(4) In the case of flame extinguishing, discharge is repeated several times at a high energy of about 1 joule using a spark plug to attempt re-ignition.
In addition to the measures described above, the use of specialized ignition devices or flame holding devices has also been attempted. For example, in a scramjet engine, a small rocket engine is provided in proximity to the inlet of the combustor, and an attempt is made to perform ignition and flame holding with this small rocket engine.
The use of plasma has also been attempted, for example. Patent Document 1 discloses an ignition device in which a high-temperature gas of several thousand degrees is blown into the combuster by using a plasma torch. Patent Document 2 discloses a rocket engine in which ignition and flame holding are performed by irradiating the combustor interior with laser light and converting the propellant into plasma gas using the heat supplied by the energy of the laser light.    [Patent Document 1] Japanese Patent Application Laid-open Publication No. 2005-171812    [Patent Document 2] Japanese Patent Application Laid-open Publication No. 2002-195151